There's nothing physically preventing them from releasing standard definition material on Bluray. The only thing holding them back is the whole "what purpose does this serve?" issue.
For something like the six TZ eps that were done on videotape, they will simply do the best they can with what they have (as noted by Mr. Laser Beam) and include them alongside the other eps for completeness' sake. One positive though is the max bitrate is higher for Bluray, so they won't have to compress the eps as much as they would on a DVD.
Also, if you read some of the stuff on the Doctor Who Restoration Team website, there's numerous mentions of Who eps that are only on videotape, but at a higher PAL resolution of 525 lines (compared to DVD's 480 lines of resolution). So there's a little bit of improvement right there.